Antiferromagnetic CuO Research Articles

Two-band conduction and nesting instabilities in superconducting Ba2CuO3+δ : First-principles study

First principles investigations of the high temperature superconducting system Ba$_2$CuO$_{3+\delta}$, recently discovered at $\delta\approx0.2$ at $T_c=70$ K, are applied to demonstrate the effects of oxygen ordering on the electronic and magnetic properties. The observed `highly over-doped' superconducting phase displays stretched Cu-planar oxygen O$_{\rm P}$ distances and anomalously shortened Cu-apical O$_{\rm A}$ separations compared with other cuprates. The stoichiometric system $\delta=0$, with its strongly one-dimensional (1D) Cu-O$_{\rm P}$ chain structure, when nonmagnetic shows 1D Fermi surfaces that lead, within density functional theory, to antiferromagnetic Cu-O$_{\rm P}$ chains (a spin-Peierls instability). Accounting for 1D fluctuations and small interchain coupling according to the theory of Schulz indicates this system, like Sr$_2$CuO$_3$, is near the 1D Luttinger-liquid quantum critical phase. The unusual Cu-O bond lengths per se have limited effects on other properties for $\delta$=0. We find that a `doubled bilayer' structure of alternating Cu-O$_{\rm P}$ chains and wide rung Cu$_3$O$_4$ ladders is the energetically preferred one of three possibilities where the additional oxygen ions bridge Cu-O$_{\rm P}$ chains in the superconducting phase $\delta=1/4$. Nominal formal valences of the three Cu sites are discussed. The six-fold (octahedral) site is the most highly oxidized, accepting somewhat more holes in the $d_{z^2}$ orbital than in the $d_{x^2-y^2}$ orbital. The implication is that two-band physics is involved in the pairing mechanism and the superconducting carriers. The Fermi surfaces of this metallic bilayer structure show both 1D and 2D strong (incipient) nesting instabilities, possibly accounting for the lack of clean single-phase samples based on this structure and suggesting importance for the pairing mechanism.

Spin transport in different oxide phases of copper

In this study, spin transport was directly compared in two types of copper oxides: The first is antiferromagnetic CuO, in which ${\mathrm{Cu}}^{2+}$ has one unpaired spin in the $3d$ orbital; the second is diamagnetic ${\mathrm{Cu}}_{2}\mathrm{O}$ with a ${\mathrm{Cu}}^{+}$ ion having a fully filled $3d$ orbital. The results indicate that CuO exhibits good spin conductivity, whereas ${\mathrm{Cu}}_{2}\mathrm{O}$ is a spin insulator. This indicates that possessing unpaired spins may be an important characteristic of good spin conductors and that copper oxides may have the potential to be spin manipulators.

Unraveling the Origin of Magnetism in Mesoporous Cu-Doped SnO₂ Magnetic Semiconductors.

The origin of magnetism in wide-gap semiconductors doped with non-ferromagnetic 3d transition metals still remains intriguing. In this article, insights in the magnetic properties of ordered mesoporous Cu-doped SnO2 powders, prepared by hard-templating, have been unraveled. Whereas, both oxygen vacancies and Fe-based impurity phases could be a plausible explanation for the observed room temperature ferromagnetism, the low temperature magnetism is mainly and unambiguously arising from the nanoscale nature of the formed antiferromagnetic CuO, which results in a net magnetization that is reminiscent of ferromagnetic behavior. This is ascribed to uncompensated spins and shape-mediated spin canting effects. The reduced blocking temperature, which resides between 30 and 5 K, and traces of vertical shifts in the hysteresis loops confirm size effects in CuO. The mesoporous nature of the system with a large surface-to-volume ratio likely promotes the occurrence of uncompensated spins, spin canting, and spin frustration, offering new prospects in the use of magnetic semiconductors for energy-efficient spintronics.

Abnormal room temperature ferromagnetism in CuO/ZnO nanocomposites via hydrothermal method

CuO/ZnO nanocomposites have been successfully synthesized by a one-step hydrothermal method with different phase ratios. Field emission scanning electron microscopy and transmission electron microscopy results show that the obtained products of nanosheets are composed of small primary particles with an average size of about 20nm. With the increasing proportion of CuO phase, nanosheets have significant collapse and the amount of small sheets increases obviously. The abnormal room temperature ferromagnetism was discovered at the interface between diamagnetic ZnO and antiferromagnetic CuO, which can be tuned by changing the phase ratios. Optical spectra indicate that the interaction between ZnO and CuO modifies the electronic structure of nanocomposites. XPS results verify the valence change of Cu ions and the presence of oxygen vacancies, which are ultimately responsible for the observed ferromagnetism. The indirect double-exchange model was employed to explain the origin of magnetism. Our study suggests that magnetically functional interfaces exhibit very appealing properties for novel devices.

Particularities of the Magnetic State of CuO Nanoparticles Produced by Low-Pressure Plasma Arc Discharge

Copper oxide nanoparticles were produced by direct plasmachemical synthesis in a plasma arc discharge of low pressure. The formation of CuO nanoparticles with an average size of 12 nm and narrow size distribution intervals was determined by using the x-ray diffraction analysis and TEM microscopy methods. It was defined by using a vibration magnetometer and a SQUID magnetometer, that the magnetic properties of CuO nanoparticles with such size were extremely different from the magnetic properties of bulk antiferromagnetic CuO. Structural defects caused the formation of a ferromagnetic state, remaining at least up to the room temperature. The temperature of corresponding antiferromagnetic ordering was significantly decreased (down to ∼ 100 K). Meanwhile, some of the copper surface spins showed a spin-glass behavior at low temperatures.

Exchange bias effect in composites of cuo nanoparticles and nanosilica glass

Nanodimensional silica based glass containing iron ions was prepared within the compressed pellet of CuO nanoparicles. The nanocomposite material showed exchange bias effect. This effect arose due to ferromagnetic iron doped CuO phase and antiferromagnetic CuO interface formation within the nanocomposite during the synthesis process. Coercive field as a function of temperature was fitted with Arhenius–Neel equation and extracted blocking temperature was 511K. The value of effective anisotropy constant for the nanocomposite was found to be 3.64x105erg/cc.

Interfacial magnetism and exchange coupling in BiFeO3–CuO nanocomposite

Ferromagnetic BiFeO3 nanocrystals of average size 9 nm were used to form a composite with antiferromagnetic CuO nanosheets, with the composition (x)BiFeO3/(100−x)CuO, x = 0, 20, 40, 50, 60, 80 and 100. The dispersion of BiFeO3 nanocrystals into the CuO matrix was confirmed by x-ray diffraction and transmission electron microscopy. The ferromagnetic ordering as observed in pure BiFeO3 occurs mainly due to the reduction in the particle size as compared to the wavelength (62 nm) of the spiral modulated spin structure of the bulk BiFeO3. Surface spin disorder of BiFeO3 nanocrystals gives rise to an exponential behavior of magnetization with temperature. Strong magnetic exchange coupling between the BiFeO3 nanocrystal and the CuO matrix induces an interfacial superparamagnetic phase with a blocking temperature of about 80 K. Zero field and field cooled magnetizations are analyzed by a ferromagnetic core and disordered spin shell model. The temperature dependence of the calculated saturation magnetization exhibits three magnetic contributions in three temperature regimes. The BiFeO3/CuO nanocomposites reveal an exchange bias effect below 170 K. The maximum exchange bias field HEB is 1841 Oe for x = 50 at 5 K under field cooling of 50 kOe. The exchange bias coupling results in an increase of coercivity of 1934 Oe at 5 K. Blocked spins within an interfacial region give rise to a remarkable exchange bias effect in the nanocomposite due to strong magnetic exchange coupling between the BiFeO3 nanocrystals and the CuO nanosheets.

Magnetic study of Cu1−xMnxO (0 ≤ x ≤ 0.08) nanoparticles

Cu1−xMnxO (0 ≤ x ≤ 0.08) nanoparticles were synthesized by a coprecipitation method. Powder x ray analysis reveals that samples show a pure CuO phase for x ≤ 0.06 and the appearance of a secondary phase CuMn2O4 for x > 0.06. The unit cell volume expands with increasing Mn concentration up to x = 0.06 and remains constant on further increase in Mn concentration. The undoped sample shows the behavior of pure CuO with antiferromagnetic transitions at TN1 = 228 K and TN2 = 214 K. Inclusion of Mn into the Cu sublattice turned antiferromagnetic CuO into a ferromagnet. The origin of ferromagnetism is analyzed on the basis of different possible interactions. It is believed that the indirect coupling between Mn ions mediated by O and Cu ions helps to stabilize the ferromagnetism.

Temperature-dependent magnetic anomalies of CuO nanoparticles

Copper oxide (CuO) nanoparticles with an average size of 25 nm were prepared by a sol–gel method. A detailed study was made of the magnetization of CuO nanoparticles using a maximum field of 60 kOe for temperatures between 8 and 300 K. Antiferromagnetic CuO nanoparticles exhibit anomalous magnetic properties, such as enhanced coercivity and magnetic moments. Significantly, the magnitude of the hysteresis component tends to weaken upon increase in temperature (>8 K). In addition, a hysteresis loop shift and coercivity enhancement are observed at 8 K in the field-cooled (FC, at 50 kOe) case. It is thought that the change in hysteresis behavior is due to the uncompensated surface spins of the CuO nanoparticles. The susceptibility ( χ ) plot showed that χ varied substantially at temperatures below 12 K, and this transition is due to the exchange interactions between the neighboring atoms at the nanoscale.

Elastic-Stress Relaxation in Compacted Nanocrystalline CuO

The magnetic properties of two types of nanocrystalline antiferromagnetic CuO samples, namely, dense nanoceramics and loose powders, were studied. For nanomaterials with smaller particles, the magnetic susceptibility χ was shown to increase with a decrease in temperature T < TN. The increase in χ in both series of samples is related to the disordering of Cu2+ spins at the surfaces of nanoparticles. The magnetic properties of nanopowders characterize the properties of isolated nanoparticles. In a dense nanoceramic, the size effect is compensated for by the interaction between nanoparticles. The magnetic properties of nanoceramics are determined by elastic stresses induced by an external action. Elastic-stress relaxation results in the recovery of magnetic order and decreases the magnetic susceptibility.

Reactivity of nanocrystalline copper oxide and its modification under magnetic field

The influence of magnetic field on the rate of oxidative dehydrogenation of 2,3,5-trimetyl-1,4-hydroquinone (TMHQ) to 2,3,5-trimetyl-1,4-benzoquinone (TMBQ) with catalysts based on copper oxide nanopowders has been investigated. The initial transformation rate of TMHQ and the final product yield greatly increase after a preliminary magnetization of nanopowders. The adsorption ability of Cu-oxide-based nanopowders with respect to ammonia enhances by 1.5 after their magnetization. Physical mechanism of reactivity variation of CuO nanopowders under the influence of weak stationary external magnetic field ( H≤3 kOe) has been proposed. As a result of magnetic field influence, the ferromagnetic regions with modified spin and charge state have appeared in an antiferromagnetic CuO of nonstoichiometric composition. These regions can be new catalytic or adsorption centers.

Mid-infrared Spectrum of Dilute Holes in Antiferromagnetic CuO2layers

The optical absorption spectrum from 0.1 to 1.5 eV photon energy is presented for extremely lightly oxygen doped crystals of La 2 CuO 4 . Crystals grown by two different methods have different low temperature spectra and different temperature dependencies. However, at room temperature the spectra are nearly identical, suggesting that they result from the optical absorption of free isolated holes in the two dimensional antiferromagnetic CuO 2 square lattice.

Does the Heisenberg Model Describe the Multimagnon Spin Dynamics in Antiferromagnetic CuO Layers?

We compute the absorption spectrum for multimagnon excitations assisted by phonons in insulating layered cuprates using exact diagonalization in clusters of up to 32 sites. The resulting line shape is very sensitive to the underlying magnetic Hamiltonian describing the spin dynamics. For the usual Heisenberg description of undoped Cu-O planes we find, in accordance with experiment, a two-magnon peak followed by high energy sidebands. However the relative weight of the sidebands is too small to reproduce the experiment. An extended Heisenberg model including a sizable four-site cyclic exchange term is shown to be consistent with the experimental data.

Suppression of Antiferromagnetic Order by Light Hole Doping inLa2Cu1−xLixO4: AL139aNQR Study

^{139}La nuclear quadrupole resonance measurements in lightly doped La_2Cu_{1-x}Li_xO_4 have been performed to reveal the dependence of the magnetic properties of the antiferromagnetic CuO_2 planes on the character of the doped holes and their interactions with the dopant. A detailed study shows that the magnetic properties are remarkably insensitive to the character of the dopant impurity. This indicates that the added holes form previously unrecognized collective structures.

Pressure dependence of the nuclear resonance of antiferromagnetic CuO.

The pressure dependence of the $^{63}\mathrm{Cu}$ NMR of a single crystal of CuO has been measured in the antiferromagnetic state at 77 K. The magnitude of the effective magnetic field at the nucleus (${\ensuremath{\nu}}_{\mathit{L}}$, the Larmor frequency) was found to decrease with pressure, \ensuremath{\partial} ln${\ensuremath{\nu}}_{\mathit{L}}$/\ensuremath{\partial}P=-1.39 ${\mathrm{Mbar}}^{\mathrm{\ensuremath{-}}1}$. The electric-field gradient (${\mathit{q}}_{\mathit{z}\mathit{z}}^{\mathrm{tot}}$) at the nucleus increases such that \ensuremath{\partial} ln${\mathit{q}}_{\mathit{z}\mathit{z}}^{\mathrm{tot}}$/\ensuremath{\partial}P=5.3 ${\mathrm{Mbar}}^{\mathrm{\ensuremath{-}}1}$, while the value calculated for external point charges is -2.7 ${\mathrm{Mbar}}^{\mathrm{\ensuremath{-}}1}$. The observed value of \ensuremath{\partial} ln${\mathit{q}}_{\mathit{z}\mathit{z}}^{\mathrm{tot}}$/\ensuremath{\partial}P must therefore arise from a redistribution of electrons over the copper 3p and 3d orbitals.

Mössbauer spectroscopy of CuO and its relevance to high-temperature superconductors.

Moessbauer emission spectroscopy and magnetic susceptibility studies were performed on {sup 57}Co-doped antiferromagnetic CuO. Magnetic susceptibility data show that antiferromagnetic ordering occurs at {ital T}{sub {ital N}}=225 K and that the Co dopant (10--100 ppm) does not influence the bulk magnetic properties of CuO. Moessbauer spectra recorded at {ital T}{gt}244 K show a dominant quadrupole-split site and two minor impurity sites. At {ital T}=244 K, a line broadening of the dominant site is observed indicating the onset of local magnetic order. The hyperfine interaction parameters at {ital T}{lt}{ital T}{sub {ital N}} were determined. In particular, the sign and the asymmetry parameter {eta} of the electric-field gradient ({ital V}{sub {ital z}{ital z}}) and the angle {theta} formed by {ital V}{sub {ital z}{ital z}} and magnetic hyperfine field were derived. We find {eta} to be very small ({eta}{le}0.09) and 21{degree}{le}{theta}{le}28{degree}. The value of {theta} is in accord with neutron-diffraction results. The {sup 57}Fe Moessbauer spectroscopy of CuO is discussed in relation to other copper oxides and Fe containing perovskites.

Gd 3+ electron paramagnetic resonance in the quasi-two-dimensional antiferromagnet YBa 2Cu 3O 6.02:Gd

Electron paramagnetic resonance (EPR) at 9.25 GHz of Y 1− x Gd x Ba 2Cu 3O 6.02 (0.001 < x < 0.05) powder magnetically a ligned along the crystallographic c-direction shows a well resolved Gd 3+ fine structure. The crystalline field parameters are determined and their temperature dependence discussed. Due to the special symmetry the exchange interaction between Gd 3+ and antiferromagnetic CuO 2 layers is small, contrary to Y 2− u Gd u BaCuO 5 where Gd 3+ and Cu 2+ magnetizations are strongly coupled. EPP serves as a probe for internal fields H I at the Gd sites; in Y 0.99Gd 0.01Ba 2Cu 2.97Fe 0.03O 6.02 H I<15 G despite the Fe-induced magnetic transition.